Follicle architecture and innervation of functionally distinct rat vibrissae

Abstract

The vibrissa follicle is a complex mechanotransducer with intricate accessory structures such as vibrissa, ring sinus and ringwulst as well as rich innervation by diverse afferent types. Establishing how afferent types and accessory structures operate together to derive specific kinds of sensory information has been challenging, because we often lack precise information on afferent types, accessory structures and vibrissa function. Here we address this challenge by synchrotron X-ray imaging of vibrissa follicles of rat vibrissae with distinct function. Specifically, we characterize accessory structures and trace myelinated axons of the all-purpose-sensing C2-, an object-sensing micro-, the wind-sensing supraorbital- and the ground-sensing trident-vibrissa. We find that while vibrissa length and follicle size differ widely across these vibrissae, the ringwulst and the associated club-like afferents are of near constant diameter and height and appear to form a non-scalable sensory module. The two longer vibrissae (supraorbital and C2 vibrissa) have noticeably more club like afferents, suggesting a special role of the ringwulst in transducing presumably smaller deflection angles encountered by long sensory hairs. The trident vibrissa receives overall few afferents, which are strongly polarized to the posterior vibrissa-shaft, a putative specialization to sensing forward-egomotion. We conclude that high-resolution structural analysis allows relating follicle architecture and function.

Fig. 1. Overview and position of functionally distinct rat vibrissae.

a MicroCT volume rendering of an iodine-stained rat head. The functionally distinct vibrissae are highlighted in color, including the large supraorbital (lSO; blue), C2 (orange), submandibular trident (trident; red), and Micro-vibrissa (micro; green). b Semi-transparent renderings of the obtained synchrotron X-ray volumes of the respective vibrissa follicles. Follicle outlines are shown as colored shades. c Cross-section through the respective image volumes at the level of the nerve entrance. Boxes mark deep vibrissal nerve axons that are shown at higher magnification in the colored insets.

Fig. 2. Hair length differences, different-sized follicles, and near-constant-size ringwulst of functionally distinct vibrissae.

a Silhouettes of trident, micro, lSO, and C2 vibrissa. b 3D segmentation of overall follicle architecture across functionally distinct vibrissae. Capsule is shown in transparent gray, ring sinus in red, inner conical body in cyan, shaft and outer root sheath in dark gray, ringwulst in yellow, and afferent innervation in multicolor (deep vibrissal innervation enters the follicle at 1/3 height and the superficial nerves enter the follicles apically). Follicles differ markedly in size. c Relative positioning of accessorial structures along the relative follicle height. Color conventions as in (b). d Top and side views of ringwulst volume renderings shown at the same scale. Ringwulst orientation has been aligned across follicles. Note the similar size of the ringwulst across different follicles. e Sizes of ring sinus, ringwulst, capsule, hair, and inner conical body across follicles relative to the size of the respective structure in the trident follicle. Sizes refer to the largest diameter of the structures.

Fig. 3. Overview of myelinated afferent innervation patterns across different follicles.

a Volume rendering of reconstructed deep vibrissal nerve myelinated afferent axons (random multicolor assignment) per follicle. Horizontal lines around the nerve bundles indicate the plane of the nerve cross-section in (b). b Cross-section of segmented myelinated axons in the nerve entering the follicles as indicated by colored boxes in (a). Dot size is proportional to fiber diameter. c Bar plots of myelinated axon counts (n = 3 counts for all four follicles) (one-way ANOVA: F  =  46.33, p < 0.001; significance according to Dunn’s Bonferroni corrected pairwise comparison with alpha = 0.05). d Boxplots of fiber diameter (axon and myelin sheath) per follicle. Asterisk indicates significant differences (one-way ANOVA: F  =  88.82, p < 0.001; significance according to Dunn’s Bonferroni corrected pairwise comparison with alpha = 0.05). Boxplots display the 25th to 75th percentile range as the box and the median as the center line. Boxplot whiskers extend by the interquartile range. Dots show individual data points. e Cumulative distribution of axon endings along the relative follicle height (%). f Fiber diameter plotted against relative terminal height in the follicle across the four vibrissae. Color conventions as in (a–e). P and r value refer to linear regression (black line).

Fig. 4. Types and endings of myelinated afferents across different vibrissae.

a Ring sinus-level Merkel- and lanceolate-endings (blue and orange respectively) across different vibrissae. b Club-like endings terminating adjacent to the ringwulst are shown in green. Not the relatively higher abundance of club-like endings and even tiling in the C2 follicle. c Below, ringwulst Ruffini-endings are shown in red. d Bar plot showing relative composition of afferent types across different vibrissa follicles.

Fig. 5. Polarization of afferent innervation differs across vibrissae.

a Volume rendering showing asymmetric innervation (blue) and ringwulst (yellow) across different vibrissae as indicated above. b Polar histograms showing innervation polarization toward the ringwulst aperture across different vibrissae. c Polarization index measured by the relative amount of afferents (%) on the ringwulst aperture side. Trident in red, micro in green, lSO in blue, and C2 in orange.